**3.1. Stable isotope profiles and trophic levels**

The values of *δ*15*N* and *δ*13*C* (mean ± standard deviation) found here are consistent to those reported in Galapagos sea lion pups (i.e., 13.1‰ ± 0.5‰ for *δ*15*N*, and -14.5‰ ± 0.5‰ for *δ*13*C*) in a recent study [31].No significant relationship was observed between isotopic values and length of the pups (*δ*15*N*: *r* = 0.005, *p* =0.7594; *δ*13*C:r* = 0.18, *p =* 0.0626) or weight (*δ*15*N:r* = 0.0001, *p* =0.9645; *δ*13*C*: *r* = 0.18, *p =* 0.0752). Although female pups appeared to exhibit higher values of *δ*15*N* compared to male pups (*t*-test = 2.3767, *p* = 0.0288), *δ*13*C* values between males and females were similar (*t*-test = -0.3326, *p* = 0.7433). In addition, no significant inter-site differences in *δ*15*N* (ANOVA, *p* = 0.4235) and *δ*13*C* (ANOVA, *p* = 0.8378) values were found among rookeries. This indicates that site or foraging location had minimal influence on the isotope ratios. The lack of differences was further minimized by sampling similar ontogenetic stages (i.e., pups of similar age, development and size), and a metabolically inactive tissue (i.e., fur hair), which is corroborated by the fact that hair is an inert tissue containing physiological and dietary information (isotopic signals) [48].

Based on the *δ*15*N*values, the trophic level (TL) measured here for the Galapagos sea lion (*δ*15*N* = 13.0; TL = 4.2) fall within the range of those recently reported (i.e., *δ*15*N* =12.6−13.4; TL = 4.1−4.4) elsewhere [30, 31, 43]. The *δ*15*N* values for thread herrings and mullets were 9.4‰ ± 1.77‰ (TL = 3.1), and 12.7‰ ± 1.10‰ (TL = 4.1), respectively, while the *δ*13*C* values for thread herrings and mullets were -17.0 ±0.70 and -9.34 ±0.80.
